When writing an image onto a photoreceptor drum, a positional deviation in a sub-scanning direction may undergo correction. Hitherto, there has been known an image forming device which compensates a positional deviation in each lens unit with an LED (light emitting diode) array. The positional deviation may be caused by the LED head being deformed and curved as a result of temperature changes. The LED head with an image forming lens may be deformed, for example, by flexing of the lens. A known method for correcting a printing position of an electrophotographic apparatus includes measuring and correcting a positional deviation of the trailing exposure that is caused by deformation.
When correcting a distortion and an inclination of the LED head, an image forming device reflects the distorted state obtained from the LED head or the inclined state obtained according to a positioning control as the correction amount. The correction amount means a delay amount of irradiation timing of the LED head and it is represented by a value indicating how many lines are to be delayed in a sub-scanning direction on the outer peripheral surface of the drum. The image forming device stores in advance correction amount values (for example, 0, 1, and −1) that correspond to every position of a plurality of light emitting segments forming the LED head, and the correspondence relation of the positions and the correction amounts as a profile for the lens unit. An exposure device reads each correction amount in every position of the plural light emitting segments arranged in a main scanning direction and delays the irradiation timing of the respective light emitting segments depending on the respective correction amounts.
However, when correcting for distortion using the correction amount for every position of the light emitting segments stored in advance, the correction amounts may locally change in the main scanning direction, creating a problem that the irregularity of an image becomes significant. On a curve of the profile, a change of the correction amount locally occurs in a range of some positions. A significant change in the size of the correction amount may occur at a position, compared to the correction amounts at the forward and backward positions. For example, in the exposure of one straight line in parallel to the main scanning direction, a deviation in a sub-scanning direction partially occurs along this straight line. Any deviation of an upper or lower step from the straight line causes the image to become visually defective.